The GNU GRUB Manual 2.x handbook can be found under
https://www.gnu.org/software/grub/manual/grub/grub.html

The default variables a in PCLinuxOS install are:

<b>GRUB_CMDLINE_LINUX_DEFAULT</b>
Unless ‘GRUB_DISABLE_RECOVERY’ is set to ‘true’, two menu entries will be generated for each Linux kernel: one default entry and one entry for recovery mode. This option lists command-line arguments to add only to the default menu entry, after those listed in ‘GRUB_CMDLINE_LINUX’.

<b>GRUB_DISABLE_OS_PROBER</b>
The grub-mkconfig has a feature to use the external os-prober program to discover other operating systems installed on the same machine and generate appropriate menu entries for them. It is disabled by default since automatic and silent execution of os-prober, and creating boot entries based on that data, is a potential attack vector. Set this option to ‘false’ to enable this feature in the grub-mkconfig command.

<b>GRUB_DISABLE_RECOVERY</b>
If this option is set to ‘true’, disable the generation of recovery mode menu entries.

<b>GRUB_TERMINAL_OUTPUT</b>
Select the terminal output device. You may select multiple devices here, separated by spaces.

Valid terminal output names depend on the platform, but may include ‘console’ (native platform console), ‘serial’ (serial terminal), ‘serial_{port}’ (serial terminal with explicit port selection), ‘gfxterm’ (graphics-mode output), ‘vga_text’ (VGA text output), ‘mda_text’ (MDA text output), ‘morse’ (Morse-coding using system beeper) or ‘spkmodem’ (simple data protocol using system speaker).

<b>GRUB_DISABLE_SUBMENU</b>
Normally, grub-mkconfig will generate top level menu entry for the kernel with highest version number and put all other found kernels or alternative menu entries for recovery mode in submenu. For entries returned by os-prober first entry will be put on top level and all others in submenu. If this option is set to ‘y’, flat menu with all entries on top level will be generated instead. Changing this option will require changing existing values of ‘GRUB_DEFAULT’, ‘fallback’ (see fallback) and ‘default’ (see default) environment variables as well as saved default entry using grub-set-default and value used with grub-reboot.

<b>GRUB_DISTRIBUTOR</b>
Set by distributors of GRUB to their identifying name. This is used to generate more informative menu entry titles.

<b>GRUB_GFXMODE</b>
Set the resolution used on the ‘gfxterm’ graphical terminal. Note that you can only use modes which your graphics card supports via VESA BIOS Extensions (VBE), so for example native LCD panel resolutions may not be available. The default is ‘auto’, which tries to select a preferred resolution.

<b>GRUB_GFXPAYLOAD_LINUX</b>
Set to ‘text’ to force the Linux kernel to boot in normal text mode, ‘keep’ to preserve the graphics mode set using ‘GRUB_GFXMODE’, ‘widthxheight’[‘xdepth’] to set a particular graphics mode, or a sequence of these separated by commas or semicolons to try several modes in sequence.

<b>GRUB_TIMEOUT</b>
Boot the default entry this many seconds after the menu is displayed, unless a key is pressed. The default is ‘5’. Set to ‘0’ to boot immediately without displaying the menu, or to ‘-1’ to wait indefinitely.

<b>GRUB_THEME</b>
Set a theme for use with the ‘gfxterm’ graphical terminal.

<b>GRUB_DEFAULT</b>
The default menu entry. This may be a number, in which case it identifies the Nth entry in the generated menu counted from zero, or the title of a menu entry, or the special string ‘saved’. Using the id may be useful if you want to set a menu entry as the default even though there may be a variable number of entries before it.

<b>GRUB_SAVEDEFAULT</b>
If this option is set to ‘true’, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. This is only useful if ‘GRUB_DEFAULT=saved’; it is a separate option because ‘GRUB_DEFAULT=saved’ is useful without this option, in conjunction with grub-set-default. Unset by default. 
